Nanofibril materials for highly sensitive and selective sensing of amines

ABSTRACT

A sensory material with high sensitivity, selectivity, and photostability has been developed for vapor probing of organic amines. The sensory material is a perylene-3,4,9,10-tetracarboxyl compound having amine binding groups and the following formula 
     
       
         
         
             
             
         
       
         
         
           
             where A and A′ are independently chosen from N—R1, N—R2, and O such that both A and A′ are not O, and R1 through R10 are amine binding moieties, solubility enhancing groups, or hydrogen such that at least one of R1 through R10 is an amine binding moiety. This perylene compound can be formed into well-defined nanofibers. Upon deposition onto a substrate, the entangled nanofibers form a meshlike, highly porous film, which enables expedient diffusion of gaseous analyte molecules within the film matrix, leading to a milliseconds response for vapor sensing.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/696,952, filed Jan. 29, 2010, and issued as U.S. Pat. No. 8,486,708,which claims the benefit of U.S. Provisional Application 61/148,780,filed Jan. 30, 2009 which are each incorporated herein by reference.

GOVERNMENT INTEREST

This invention was made with government support under Grants CHE0641353and CBET730667 awarded by the National Science Foundation. TheGovernment has certain rights to this invention.

FIELD OF THE INVENTION

This invention relates generally to fluorescent materials, and the useof such materials for detection of amines. Therefore, the presentinvention relates generally to the fields of chemistry and materialsscience.

BACKGROUND OF THE INVENTION

Development of sensors or probes that can be used to detect the tracevapor of organic amines represents one of the active research fields inchemistry and materials science, particularly those related to theemerging nanoscience and nanotechnology. Volatile amines have beenheavily used in various areas ranging from chemical and pharmaceuticalto food industries. Some of the amines, like hydrazine, have also beenused in the military as fuel additives in rocket and fighter jetpropulsion systems. Detecting these amines with high sensitivity is notonly critical to air pollution monitoring and control but also mayprovide expedient ways for quality control of food and even medicaldiagnosis of certain types of disease. For example, in diagnosing uremiaand lung cancer, released biogenic amines are commonly used asbiomarkers.

Although much success has been achieved for detection of amines insolutions using various types of sensors, the vapor-based detection ofgaseous amines still remains challenging. This challenge is largely dueto the limited availability of sensory materials that enable vapordetection with both high sensitivity and selectivity. Fluorescentsensing and probing based on organic sensory materials represents aunique class of detection techniques that usually provide a simple,expedient way for chemical detection and analysis. However, there arenot many organic materials available that are sufficiently fluorescentin the solid state and suited for use as sensory materials in vapordetection. These materials may be strongly fluorescent in molecularstate in solutions. Moreover, compared to the more common p-type (i.e.,electron donating) materials, which are suited for sensing oxidativereagents like nitro-based compounds, the availability of n-type organicmaterials (i.e., electron accepting, and suited for sensing reducingreagents like amines) is much more limited.

SUMMARY

In light of the problems and deficiencies noted above, amines sensorassemblies can include a porous film of entangled nanofibers on asubstrate. The nanofibers can include 3,4,9,10-tetracarboxyl perylenecompounds having the formula I:

where A and A′ are independently chosen from N—R1, N—R2, and O such thatboth A and A′ are not O, and R1 through R10 are amine binding moieties,solubility enhancing groups, or hydrogen such that at least one of R1through R10 is an amine binding moiety.

A nano fiber-based sensor compound can be formed via synthesis of theunderlying perylene compound which is then formed into the nanofibers.For example, a 3,4,9,10-tetracarboxyl perylene compound having theFormula I (as previously noted) can be synthesized. The perylenecompound can be self-assembled into nanofibers via any suitable processsuch as, but not limited to, a slow controlled solvent-exchange step,rapid solution dispersion, phase transfer at the interface between twosolvents, sol-gel processing, direct vaporization of the solvent, or anyother suitable self-assembly methods including the surface assistedprocess. The nanofiber fluorescent sensor compound can optionally beformed into a film of entangled nanofibers by coating the nanofiberdispersion on a substrate.

There has thus been outlined, rather broadly, the more importantfeatures of the invention so that the detailed description thereof thatfollows may be better understood, and so that the present contributionto the art may be better appreciated. Other features of the presentinvention will become clearer from the following detailed description ofthe invention, taken with the accompanying figures and claims, or may belearned by the practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully apparent from the followingdescription and appended claims, taken in conjunction with theaccompanying drawings. Understanding that these drawings merely depictexemplary embodiments of the present invention and they are, therefore,not to be considered limiting of its scope. It will be readilyappreciated that the components of the present invention, as generallydescribed and illustrated in the figures herein, could be arranged,sized, and designed in a wide variety of different configurations.Nonetheless, the invention will be described and explained withadditional specificity and detail through the use of the accompanyingdrawings in which:

FIG. 1 is a schematic illustration of a slow solvent evaporation systemused in accordance with one embodiment of the present invention.

FIG. 2 shows (A) SEM image of a nanofibril film deposited on a glassslide. (B) Zoom-in SEM image of the nanofibril film. (C, D) Bright-fieldand fluorescence optical microscopy image of a nanofibril film. Notethat due to the diffraction effect the fiber in the optical microscopyimage appears larger than the real size as measured by SEM.

FIG. 3A is a fluorescence quenching efficiency (1−I/I₀) as a function ofthe vapor pressure of aniline: data (error 5%) fitted with the Langmuirequation.

FIG. 3B is a fluorescence spectra of a nanofibril film before (red) andafter (blue) exposure to the saturated vapor of aniline (880 ppm) for 10s.

FIG. 3C shows the absorption (black) and fluorescence (red) spectra ofmolecule 1 in chloroform solution (dashed) and the nanofiber state(solid). The raised baseline for the absorption spectrum of nanofibrilfilm is primarily due to the light scattering.

FIG. 4 shows energy levels of HOMO (π) and LUMO (π*) orbitals of 1 andaniline showing the favorable electron transfer from amine to thephotoexcited state of 1. The same diagram applies to the other amines,while the reducing power (or the π-orbital level) would be differentfrom that of aniline (see Table 1). Geometry optimization and energycalculation were performed with density-functional theory(B3LYP/6-311g**//B3LYP/6-31g*) using Gaussian 03 package.

FIG. 5 is a comparison between the fluorescence spectra of thenanofibril film of 1 (black) and a thin film (red) drop-cast from theTHF solutions of a PTCDI molecule modified by two bulky, branchedside-chains,N,N′-di(hexylheptyl)-perylene-3,4,9,10-tetracarboxyl-diimide (HH-PTCDI),which forms ill-shaped aggregates, mainly due to the significant sterichindrance caused by the large side-chains.

FIG. 6 is a time-course of fluorescence quenching of a nanofibril filmupon blowing over with saturated vapor of aniline (880 ppm), indicatinga response time of about 0.32 s. The intensity was monitored at 628 nm.

FIG. 7A is a fluorescence spectra of a nanofibril film before (black)and after (red) exposure to diluted vapor of aniline

FIG. 7B is a fluorescence spectra of a nanofibril film before (black)and after (red) exposure to diluted vapor of hydrazine.

FIG. 8 is a fluorescence spectra of a nanofibril film after continuousirradiation at 550 nm for 0, 10, 20, 30, 60 min. The film was held inthe LS55 fluorometer with a constant excitation slit of 5 nm and apulsed Xenon discharge lamp (7.3 W) as the light source. The unchangedfluorescence indicates the robust photostability of the film.

FIG. 9 is a bar graph of fluorescence response of the nanofibril film tovarious organic reagents: 1, methanol; 2, acetone; 3, acetic acid; 4,THF; 5, acetonitrile; 6, chloroform; 7, toluene; 8, hexane; 9,cyclohexane; 10, nitromethane; 11, nitrobenzene; 12, phenol; 13,cyclohexylamine; 14, dibutylamine; 15, aniline; 16, butylamine (3 s);17, triethylamine; 18, hydrazine; 19, ammonium hydroxide. Unlessotherwise marked, the exposure times for the amines and all the otherreagents are 10 and 15 s, respectively.

FIG. 10 shows five continuous cycles of quenching-recovery which weretested for a nanofibril film upon exposure to the saturated vapor ofphenol (335 ppm). The quenching was performed by exposing the film tothe phenol vapor for 15 s. After each cycle of quenching, thefluorescence of the film was recovered by exposing the film to an openair for 60 min or at an elevated temperature (60° C.) for 5 min.

FIG. 11 (A) SEM image of the nanofibers deposited on a glass slide. (B)Fluorescence optical microscopy image of a nanofibril film deposited ona glass slide.

FIG. 12 (A) Fluorescence spectra of a nanofibril film after 60 s ofexposure to aniline vapor at 35, 70, 175, 350, 525, 875, 1750 ppb. (B)Fluorescence spectra of a nanofibril film measured 15 min (red) and 30min (blue) after the complete quenching (shown in black) in the presenceof 1750 ppb aniline vapor. No spectral change with time indicatesirreversibility of the quenching process. The same test was alsoperformed with the larger fibers (350 nm in diameter) as shown in theinset, where significant recovery of the fluorescence emission wasobserved.

FIG. 13 (A) Fluorescence quenching efficiency (1−I/I₀) as a function ofthe vapor concentration aniline, measured for the nanofibril filmsdeposited from both the ultrathin nano fibers (30-50 nm) and largefibers (350 nm); comparative investigation was performed on threedifferent films fabricated from the ultrathin nanofibers using variousamount of fibril materials (cyan: 0.35 mg; blue: 0.15 mg; red: 0.1 mg)as well as a film fabricated from the large fibers (black: 0.35 mg). (B)Fitting the three sets of data in (A) with the Langmuir equation aimingto predict the detection limit based on the common photon detectionthreshold of PMT. All data are with an error of ±3%.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following detailed description of exemplary embodiments of theinvention makes reference to the accompanying drawings, which form apart hereof and in which are shown, by way of illustration, exemplaryembodiments in which the invention may be practiced. While theseexemplary embodiments are described in sufficient detail to enable thoseskilled in the art to practice the invention, it should be understoodthat other embodiments may be realized and that various changes to theinvention may be made without departing from the spirit and scope of thepresent invention. Thus, the following more detailed description of theembodiments of the present invention is not intended to limit the scopeof the invention, as claimed, but is presented for purposes ofillustration only and not limitation to describe the features andcharacteristics of the present invention, to set forth the best mode ofoperation of the invention, and to sufficiently enable one skilled inthe art to practice the invention. Accordingly, the scope of the presentinvention is to be defined solely by the appended claims.

DEFINITIONS

In describing and claiming the present invention, the followingterminology will be used.

The singular forms “a,” “an,” and “the” include plural referents unlessthe context clearly dictates otherwise. Thus, for example, reference to“a binding moiety” includes reference to one or more of such groups andreference to “exposing” refers to one or more such steps.

As used herein, “alkylene” refers to a saturated hydrocarbon having twovalencies, i.e. for bonding with adjacent groups. Non-limiting examplesof alkylenes include —CH—, —CH₂—, —C₂H₄—, —C₃H₆—, etc. This is incontrast to “alkyl” groups which are similar but have a single valencyand include at least one CH₃ end group.

As used herein, when referring to a component of a composition,“primarily” indicates that that component is present in a greater amountthan any other component of the relevant composition.

As used herein with respect to an identified property or circumstance,“substantially” refers to a degree of deviation that is sufficientlysmall so as to not measurably detract from the identified property orcircumstance. The exact degree of deviation allowable may in some casesdepend on the specific context.

As used herein, a plurality of items, structural elements, compositionalelements, and/or materials may be presented in a common list forconvenience. However, these lists should be construed as though eachmember of the list is individually identified as a separate and uniquemember. Thus, no individual member of such list should be construed as ade facto equivalent of any other member of the same list solely based ontheir presentation in a common group without indications to thecontrary.

Concentrations, amounts, and other numerical data may be presentedherein in a range format. It is to be understood that such range formatis used merely for convenience and brevity and should be interpretedflexibly to include not only the numerical values explicitly recited asthe limits of the range, but also to include all the individualnumerical values or sub-ranges encompassed within that range as if eachnumerical value and sub-range is explicitly recited. For example, anumerical range of about 1 to about 4.5 should be interpreted to includenot only the explicitly recited limits of 1 to about 4.5, but also toinclude individual numerals such as 2, 3, 4, and sub-ranges such as 1 to3, 2 to 4, etc. The same principle applies to ranges reciting only onenumerical value, such as “less than about 4.5,” which should beinterpreted to include all of the above-recited values and ranges.Further, such an interpretation should apply regardless of the breadthof the range or the characteristic being described.

Any steps recited in any method or process claims may be executed in anyorder and are not limited to the order presented in the claims unlessclearly indicated otherwise. Means-plus-function or step-plus-functionlimitations will only be employed where for a specific claim limitationall of the following conditions are present in that limitation: a)“means for” or “step for” is expressly recited; and b) a correspondingfunction is expressly recited. The structure, material or acts thatsupport the means-plus function are expressly recited in the descriptionherein. Accordingly, the scope of the invention should be determinedsolely by the appended claims and their legal equivalents, rather thanby the descriptions and examples given herein.

Fluorescent Sensor Compounds for Detecting Amines

A new type of fluorescence sensor for expedient vapor detection oforganic amines with both high sensitivity and selectivity is provided.The sensing mechanism is primarily based on quenching of thefluorescence emission of the sensory materials upon interaction with theamine molecules. The sensory materials can be composed of well-definednanofibers fabricated from an n-type organic semiconductor molecule. Thelong-range exciton migration intrinsic to the one-dimensionalwell-organized molecular arrangement within the nanofiber enablesamplified fluorescence quenching by the surface adsorbed analytes(quencher molecules). Upon deposition onto a substrate, the entanglednanofibers form a mesh-like, highly porous film, which provides maximaladsorption and accumulation of the gaseous molecules under detection,leading to expedient vapor sensing of amines with unprecedentedefficiency (down to detection limit in ppt range).

Fluorescent sensor compounds for detecting amines can be3,4,9,10-tetracarboxyl perylene compounds can generally have the formulaI:

where A and A′ are independently chosen from N—R1, N—R2, and O such thatboth A and A′ are not O, and R1 through R10 are amine binding moieties,solubility enhancing groups, or hydrogen such that at least one of R1through R10 is an amine binding moiety. Typically, the fluorescentsensor compounds can be formed into a nanofiber structure although thisis not required.

In one specific aspect, the fluorescent sensor compound can be animide-anhydride perylene where A is N—R1 and A′ is O. Formula IIillustrates one specific class of imide-anhydride perylenes where R3—R6and R7—R10 are hydrogen.

In this case, the anhydride moiety (O═C—O—C═O) is an amine bindingmoiety which does not have steric hindrance sufficient to disruptformation of one-dimensional self-assembly of the compound into ananofibril structure of the present invention. The group R1 can bechosen to provide solubility of the compound in the organic solvent andwhich also does not disrupt self-assembly into a nanofibril structure.Such disruption may not be undesirable if nanofibrils are not theintended final product morphology. In one aspect, R1 is a C1 to C13alkyl chains which can be straight or branched. Non-limiting examples ofbranched alkyls for R1 can include symmetric branched alkyls such ashexylheptyl, pentylhexyl, and butylpentyl. However, asymmetric branchedalkyls can also be suitable such as butylheptyl, 4-methyl-1-hexylheptyl,and the like. As a general rule smaller alkyl chains such as methyethyland propylbutyl tend to exhibit low solubility, depending on theparticular molecule.

In another alternative, the 3,4,9,10-tetracarboxyl perylene compound canbe a bisimide, i.e. A is N—R1 and A′ is N—R2. Each of R1 and R2 can beC1 to C13 alkyl groups as discussed above. Furthermore, carboxylic acidcan be a side group which is added to act as the amine binding group.Formula III illustrates one alternative class of carboxylic acidbisimides of the present invention.

The solubility enhancing groups can be oriented as side groups (R3-R10)or as in Formula III at R1 to control or increase solubility of thecompound during manufacture of nanofibers Although other solubilityenhancing groups (R1) can be suitable as outlined herein, one embodimentof formula III can include symmetric alkyl groups such as, but notlimited to, hexylheptyl, pentylhexyl, and butylpentyl.

In still another alternative embodiment, the 3,4,9,10-tetracarboxylperylene compound can include carboxylic acid and/or anhydride moieties.Such side groups can be useful to provide amine binding groups. Asdiscussed in more detailed below, such solution processing usuallyinvolves self-assembly mechanisms. In some embodiments, the solubilityenhancing moieties can be located along the sides of the perylene core,i.e. R3-R6 and R7-R10. However, most often the amine binding moietiescan be located along sides of the perylene core. Formulas IV-VIillustrate several carboxylic acid and anhydride substituted perylenecompounds suitable for use in the sensor compounds.

Formula IV illustrates a compound having a maleic anhydride moietyformed collectively of R4 and R5. Formula V and IV illustrate3,4,9,10-tetracarboxyl perylene compounds which include carboxylic acidsas side groups, although almost any combination or number of R3 throughR10 can be COOH, one or two carboxylic acid groups are most typical. TheR1 and R2 end groups can be chosen from among those previously listed.However, C5-C12 cycloalkyls can also be employed as the side-chainssubstituted at the imide position (A or A′) as the solubility enhancinggroups to facilitate solution processing. These cycloalkyl groups aresuitable for one-dimensional self-assembly of the molecules intonanofibrils. For example, non-limiting examples of cycloalkyls caninclude cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, andcyclododecyl. Specific examples of amine binding sites can include anoxygen moiety like anhydride or an acid like —COOH. While many of theamine-binding moieties mentioned above can be substituted at the bayarea, some of the bulky alkyl groups like the branched ones aregenerally not suitable for substitution at the bay area when formingnanofibers, since they will distort the pi-pi stacking between theperylene planes mainly due to the increased steric hindrance.

The nanofiber-based fluorescent sensor compounds can be formed viasynthesis of the underlying perylene compound which is then formed intothe nanofibers. For example, a 3,4,9,10-tetracarboxyl perylene compoundhaving the Formula I (as previously noted) can be synthesized. In onespecific example, the starting compound used for synthesizing the sensormolecule, 3,4,9,10-tetracarboxylic perylene dianhydride (Formula I withA and A′ both as O, and R3-R10 as hydrogen) can be obtained commerciallyfrom many chemical manufacturers including Sigma Aldrich and FisherScientific. A diimide compound synthesized from the dianhydride can besubjected to partial hydrolysis to form an anhydride imide such as thosedescribed by Formula II above.

The perylene compound can be self-assembled into nanofibers via anysuitable process such as, but not limited to, a slow controlledsolvent-exchange step, rapid solution dispersion, phase transfer at theinterface between two solvents, sol-gel processing, direct vaporizationof the solvent, or any other suitable self-assembly methods includingthe surface assisted process. A more detailed description of some ofthese options can be found in a recent publication, Ling Zang, Accountsof Chemical Research, a special issue in Nanoscience, 41 (2008)1596-1608, which is incorporated herein by reference. The slowcontrolled solvent-exchange step can be accomplished by dissolving thecompound in a suitable good solvent, e.g. dichloromethane, chloroform,tetrachloromethane, alkanes, etc., which typically have a solubility ofat least 0.2 mM and in some cases at least 1 mM concentration for theperylene compound. A solution of the perylene compound can be placed ina closed chamber in proximity to a poor solvent (e.g. some solubilityfor the perylene compound but generally less than about 1 μMconcentration and in some cases less than about 0.01 mM). Poor solventscan vary depending on the particular perylene compound but can ofteninclude methanol, ethanol, hexane, heptane, cyclohexane, acetonitrile,etc. Vapor diffusion between the two solvents will gradually decreasethe concentration of good solvent in the perylene solution and thesolubility of the solution. As a result the perylene compound begins tocrystallize slowly into the nanofibers of the present invention. Therate of nanofiber formation can depend on the particular solvents,temperature, etc., but is often about a day to reach equilibrium. Theultrathin nanofibers (20-50 nm in diameter) can be fabricated via aquick crystallization method, e.g. injecting the good solvent solutionof perylene monoimide (e.g. 0.3 mL, 3.4 mM) into poor solvent (e.g.hexane, 1.2 mL) in a small test tube, followed by 30 min aging.

The nanofibers can vary in size, depending on the specific perylenecompound used. However, as a general guideline, the nanofibers can havea diameter from about 10 nm to about 1000 nm, in some cases to about 500nm, and one aspect from about 100 nm to about 350 nm while in anotheraspect from about 10 nm to about 50 nm. Similarly, the length of thenanofibers can vary considerably but is often from about 1 μm to about 1mm, and in some cases from about 10 μm.

The formed nanofibers can then be suspended in a liquid vehicle in whichthe nanofibers are very poorly soluble, e.g. less than about 1micromolar concentration, at least less than 0.01 mM, or completelyinsoluble, to form a nanofiber dispersion. Non-limiting examples ofsuitable liquid vehicles can include hexane, heptane, methanol,cyclohexane, alcohols, and the like.

The nanofiber fluorescent sensor compound can be formed into a film ofentangled nanofibers by coating the nanofiber dispersion on a substrateand allowing the solvent to evaporate.

The formed nanofibers have shown rapid fluorescence responses uponexposure to various amine compounds. The nanofiber fluorescent sensorcompound can be exposed to a fluid sample in which the nanofiberfluorescent sensor compound is not substantially soluble. The fluidsample can generally be a fluid containing the target gaseous analyte,although liquids can also be tested. A fluorescence change can bemeasured and/or displayed upon exposure of the nanofiber sensor compoundto the fluid sample. Typically, the fluorescence change can beaccomplished using a fluorometer, or simply a photon detector that canmeasure the fluorescence emission intensity. Depending on theapplication, the displaying of fluorescence change can be a quantitativemeasure of fluorescence response, e.g. a percentage change ofluminescence intensity. Alternatively, the displaying is qualitativesuch as by visual observation of a fluorescence change. Such qualitativemeasure can be sufficient when the mere presence of a particular amineis sought rather than an absolute measure of the concentration.

The specific performance of individual perylene nanofibers can vary.However, in one aspect of the invention, the nanofiber fluorescentsensor compound can exhibit a fluorescence change (e.g. quenching) from50% to 100% for a majority of amines selected from the group consistingof phenol, cyclohexylamine, dibutylamine, aniline, butylamine,triethylamine, hydrazine, and ammonium hydroxide. Furthermore, thefluorescence change for each of cyclohexylamine, dibutylamine, aniline,butylamine, triethylamine, hydrazine, and ammonium hydroxide can mostoften be from about 80% to about 100%.

Advantageously, the nanofiber fluorescent sensor compounds can beregenerated by dissolving the nanofiber fluorescent sensor compound andregenerating the nano fibers as previously described. It is noted thatsuch regeneration also does not typically involve a chemical reaction,but rather dissolving of the perylene compound in a suitable solvent andrepeating the self-assembly process previously described. Thus, althoughnot generally regenerable by an end user, the sensor compound can bereadily collected and recycled with no residual effects on theperformance of the material.

Furthermore, the fluorescent sensor compounds can be used as fluorescentdyes or other applications such as in solar cells and the like which donot require nanofiber morphology. This technology can also find a broadrange of applications in health and security examination, where instantdetection of trace amine is usually demanded. Indeed, sensitive vapordetection of organic amines is not only critical to the air pollutionmonitoring and control, but will also provide expedient ways for foodquality control, and even medical diagnosis of certain types of disease,e.g., uremia and lung cancer, for which biogenic amines released areusually used as the biomarkers.

Compared to the electrical sensors like those based on chemiresistors,the reported fluorescent (optical) sensor system represents a class ofsimple, expedient technique for chemical vapor detection and analysis.In contrast to the polymer film-based fluorescent sensors, thenanofibril film-based sensors provide three-dimensional continuous pores(or channels) formed by the entangled piling of the nanofibers, enablingexpedient diffusion of the analyte molecules throughout the film matrix,and thus fast response (milliseconds) for the sensing. The high porosity(and thus large surface area) formed by the entangled piling ofnanofibers also provides maximal adsorption and accumulation of thegaseous molecules under detection, leading to expedient vapor sensing ofamines with unprecedented efficiency (down to detection limit in pptrange). The nanofibril materials, as well as the new sensing module thusdeveloped, can open wide options to improve the detection efficacy andfind broad range of applications in health and security examination,where instant detection of trace amine is usually demanded.

Perylene-tetracarboxylic diimide (PTCDI) represents a robust class ofn-type organic materials with strong photostability, which isparticularly desirable for being used in optical sensing or probingregarding both the performance sustainability and reproducibility. Thesensor compounds can find broad applications in health and securityexamination. For example, air quality and security industries canbenefit from real time amine detection. In-field monitoring of airquality against pollution by toxic amines is one example, which havecommonly been used in various industry and military systems.Particularly, hydrazine has been heavily used in both industry (as anoxygen scavenger and corrosion inhibitor) and military (as a fuel inrocket propulsion systems). Moreover, this compound has been implicatedas a carcinogen and is readily absorbed through the skin. Anothertypical toxic amine is ethanolamine, which has been used as thescrubbing agent in the ventilation system of submarines to remove carbondioxide from the air. Due to their toxicity and reactivity, faciledetection of these amines is relevant to both life and environmentsecurity.

Health and clinic applications can include rapid screening of uremia andlung cancer, one of the most common cancers, particularly in thedeveloping countries. Alkyl-amines will be used as the biomarkers foruremia diseases, while aromatic-amines will be used for lung cancer.Very trace amount of amines breathed out of the patient will be detected(at concentration of ppt), thus enabling rapid diagnostics or warning ofthe diseases at the early stage. Food industry applications can includehigh throughput quality control and monitoring by detecting the aminesreleased from foods.

A new type of fluorescence sensory material with high sensitivity,selectivity, and photostability has been developed for vapor detectionof organic amines. The sensory material is primarily based onwell-defined nanofibers fabricated from an n-type organic semiconductormolecule. Upon deposition onto a substrate, these entangled nanofibersform a meshlike, highly porous film, which allows for maximal exposureto the gaseous analyte molecules, expedient diffusion of the moleculesthroughout the meshlike film, and increased adsorption and accumulationof the gaseous molecules within the porous matrix.

Compared to the electrical sensors like those based on chemiresistors,the reported fluorescent sensor system represents a class of simple,expedient technique for chemical detection and analysis. In contrast tothe polymer-film-based fluorescent sensors, the nano fibril-film-basedsensors provide three-dimensional continuous pores (or channels) formedby the entangled piling of the nanofibers, enabling expedient diffusionof the analyte molecules throughout the film matrix, and thus fastresponse (milliseconds) for the sensing. The nanofibril materials, aswell as the new sensing module thus developed, may open wide options toimprove the detection efficacy and find broad range of applications inhealth and security examination, where instant detection of trace amineis highly beneficial.

Example 1

A strongly fluorescent n-type organic semiconductor material, which canbe fabricated into well-defined nanofibers and employed in efficientfluorescent probing of gaseous amines is described. Without being boundto any particular theory, it is thought that the long-range excitonmigration intrinsic to the one-dimensional well-organized moleculararrangement within the nanofiber enables amplified fluorescencequenching by the surface adsorbed analytes (quencher molecules). Takingadvantage of such amplified fluorescence quenching intrinsic tonanofibers, a new type of nanofibers was fabricated from an n-typematerial that can be used for effective sensing of reductive compounds,such as organic amines, through electron-transfer-based fluorescencequenching. The building block molecule (1) employed for the nanofibrilfabrication is shown in Formula V, which was synthesized through partialhydrolysis of hexylheptyl substituted 3,4,9,10-perylene-tetracarboxylicdiimide (PTCDI).

In particular,N-(1-hexylheptyl)perylene-3,4,9,10-tetracarboxyl-3,4-anhydride-9,10-imide(1) was synthesized by suspending 1 gN,N′-di(hexylheptyl)-perylene-3,4,9,10-tetracarboxyl-diimide (1.3 mmol)in 60 mL of t-BuOH containing 700 mg solid KOH (85%). The mixture washeated with vigorous stirring to reflux. After refluxing for 1.5 h, thereaction solution was cooled to room temperature, followed by additionof 50 mL of 2 M HCl, followed by stirring over night. The resultingsolid was collected by vacuum filtration through a 0.45 μm membranefilter (Osmonics). The solid was then washed thoroughly with distilledwater until the pH of washings turned to be neutral. The hydrolyzedproduct fromN,N′-di(hexylheptyl)-perylene-3,4,9,10-tetracarboxyl-diimide wasdirectly purified by column chromatography (eluent: methylene chloride),yielding 0.35 g (35%) of molecule 1, having the following properties:¹H-NMR (CDCl₃): δ=0.83 (t, 6H, 2CH₃), 1.17-1.42 (m, 16H, 8CH₂), 1.85 (m,2H, CH₂), 2.24 (m, 2H, CH₂), 5.19 (m, 1H, CH), 8.67 (m, 8H, perylene).

Self-assembly of molecule 1 into nanofibers was performed through a slowsolvent-exchange process, which was realized via vapor diffusion withina closed chamber. Briefly, a test tube containing about 0.2 mL CHCl₃solution of 1 (1.7 mM) was placed in a 50 mL jar, which contained about10 mL of methanol, followed by sealing the jar for slow vapor diffusionbetween the two solvents (FIG. 1). Upon gradual solvent exchange, thesolution in the test tube became more dominant with methanol, which is apoor solvent (with low solubility) for molecule 1, thereby leading toself-assembly of the molecules into nanofibers.

Because of the slow crystallization process controlled by the slow vapordiffusion, the nanofibers fabricated via such a process are usually in awell-defined shape and sizes as shown in FIG. 2A-D. After about one daythe exchange between the two solvents reached the equilibrium, resultingin complete assembly of the molecules, and precipitating down to thebottom of the test tube. The nanofibers thus obtained were re-dispersedin hexane, producing a suspension well-suited for deposition on asubstrate either for microscopy imaging or vapor sensing tests. For eachof the sensing tests, the whole amount of the nanofibers thus preparedwere deposited on a glass substrate to produce a film that maintainedthe same surface area (adsorption) for all the sensing tests aspresented in FIG. 3A.

The fluorescence quantum yield (φ) of the nanofibril film was estimatedby measuring the absorption and fluorescence intensity in comparisonwith a thin-film fluorescence standard with φ=100%. The thin-filmstandard was prepared by sandwiching one drop of a polystyrene/toluenegel between two glass cover slips. Within the gel was dissolved anappropriate concentration of a PTCDI molecule,N,N′-di(hexylheptyl)-perylene-3,4,9,10-tetracarboxyl-diimide (HH-PTCDI).By maintaining molecular dispersion of the molecules within the gel, thefluorescent quantum yield of HH-PTCDI remains 100%, as it is dissolvedin a homogeneous solution in toluene or other good organic solvents.

Molecule 1 possesses a structure that provides a good balance betweenthe molecular stacking and the fluorescence yield of the materials thusassembled. The former prefers a molecular structure with minimal sterichindrance (usually referring to a small or linear side chain), while thelatter favors bulky, branched side chains that may distort the π-πstacking to afford increased fluorescence (by enhancing the low-energyexcitonic transition) for the molecular assembly. FIG. 2A shows thescanning electron microscopy (SEM) image of the nanofibers fabricatedfrom molecule 1 through the vapor-diffusion (slow solvent exchange)process as described in FIG. 1. The average diameter of the nanofiberswas ˜350 nm as determined by zoom-in SEM imaging as shown in FIG. 2B.

The extended one-dimensional molecular arrangement obtained for molecule1 is likely dominated by the π-π interaction between the perylenebackbones (which is sterically favored by the bare end of molecule 1),in cooperation with the hydrophobic interactions between the side chainsin appropriate size. Such a molecular arrangement is reminiscent of theone-dimensional self-assembly commonly observed for detergents, lipids,or amphiphilic peptides, for which extended molecular assembly can beachieved through the concerted electrostatic and hydrophobicinteractions. It seems that one-dimensional molecular assembly ofmolecule 1 is dependent on the size of the side chains. Replacing theside chain of molecule 1 with a larger group, for example, nonyldecyl,resulted in formation of only ill-shaped molecular aggregates. Thenanofibers fabricated from molecule 1 demonstrates strong fluorescencewith yield ˜15% as depicted in the fluorescence microscopy images (FIG.2C and FIG. 2D), implying a distorted molecular stacking that is usuallyobserved for the PTCDI molecules modified with branched side chains. Thestrong red fluorescence of the nanofibers can easily be observed evenwith the naked eye, making the nanofibers more feasible to be used inqualitative fluorescence sensing. Although not essential to anunderstanding of the compounds, an illustrative movie clip shows ademonstration of one embodiment of this compound and can be found atpubs.acs.org (Supporting Information for Nano Lett., 2008, 8(8),2219-2223, which article is incorporated herein by reference) orwww.chem.siu.edu/zang/image/gas-sensor.wmv.

FIG. 3C shows the absorption and fluorescence spectra measured from thenanofibers deposited on glass substrate, in comparison to the spectrameasured for molecule 1 dissolved in a chloroform solution. Theelectronic property of molecule 1 as depicted in FIG. 3C is quitesimilar to the parent PTCDI molecules with the HOMO-LUMO gap around 2.5eV, consistent with the ab initio calculation results (FIG. 4).

TABLE 1 Physical properties and quenching results of various amines andphenol Oxidation Vapor Quenching potential^(a) Driving pressure^(b)efficiency E_(1/2) value(V) force^(a) ppm at (10 s of Analyte vs SCE ΔG(−eV) 25° C. exposure) (%) Butylamine 1.52 0.62 120400 96 Pentylamine1.69^(c) 0.45 39480 95 Hexylamine 1.72^(d) 0.42 8580 95 Octylamine — —1280 94 Dibutylamine 1.20 0.94 3360 91 Triethylamine 0.99 1.15 75990 85Cyclohexylamine 1.72^(d) 0.42 11840 94 Cyclopentylamine — — — 94 Aniline0.86 1.28 880 95 Hydrazine 0.43 1.71 5920 98 Phenol 1.37 0.77 340 54^(a)The driving force for the fluorescence quenching, i.e., photoinducedelectron transfer from the analyte to 1 was calculated using theRehm-Weller equation: ΔG = −e(E^(o) _(red) − E^(o) _(ox)) − ΔE_(oo),where E^(o) _(red) and E^(o) _(ox) are the reduction potential ofelectron acceptor and the oxidation potential of electron donor,respectively, and ΔE_(oo) is the singlet excitation energy. ^(b)Thevapor pressure data are cited from CRC handbook of Chemistry andPhysics, 85th Edition, CRC Press, 2004, p15-16 to 25. ^(c)The oxidationpotential of pentylamine (determined as the peak potential). ^(d)Theoxidation potentials of hexylamine and cyclohexylamine. The relativelylower quenching efficiency observed for the tertiary amines might be dueto the weaker chemical binding with the anhydride, with which thebinding of a tertiary amine is primarily through the donor-acceptorinteraction, but lack of hydrogen bonding.

The fluorescence quantum yield of molecule 1 in solution is ˜100%, thesame as other PTCDI molecules tested. Upon assembly into the solidstate, the fluorescence of individual molecules disappeared, while a newemission band formed at a longer wavelength centered around 628 nm.Compared to the emission spectrum (0.21 eV fwhm) obtained from theill-shaped molecular aggregate formed from the parent PTCDI moleculemodified with two hexylheptyl side chains (FIG. 5), the emissionmeasured for the nanofibers of molecule 1 exhibits a significantlynarrower band, only 0.17 eV fwhm, implying the well-organized molecularassembly within the nanofibers. Consistently, a new, pronounced band wasobserved at the longer wavelength in the absorption spectrum of thenanofibers, which is typically characteristic of the strong π-πinteraction as observed in the self-assemblies of PTCDI and other planarπ-conjugated molecules. The strong π-π interaction is also revealed bythe characteristic enhancement of the transitions (absorptions) fromground state to the higher levels of electronic states (0-1, 0-2, and0-3, compared to 0-0) of the component molecules. The strong π-πinteraction may enhance the exciton migration, which is now moreconfined along the long axis of the nanofiber, leading to amplificationin fluorescence quenching by the surface adsorbed analytes (quenchers).

Upon fabrication from hydrophilic solvents such as alcohols, thenanofibers are expected to possess a surface predominantly consisting ofthe anhydride moieties, which are more hydrophilic compared to thehexylheptyl group located at the other end of the molecule. A surfacefull of anhydride moieties enables strong chemical binding or adsorptionwith amines through both hydrogen bonding and donor-acceptor (chargetransfer) interaction. Deposition of the nanofibers onto a substrateproduces a meshlike film that is primarily formed by entangled piling ofthe fibers and thus possesses porosity on a number of length scales(FIG. 2A-D). Such a porous film not only provides increased surface areafor enhanced adsorption of gaseous molecules but also enables expedientdiffusion of guest molecules across the film matrix, leading toefficient probing of the gaseous molecules with both high sensitivityand fast time response.

The organic compounds employed for sensing tests include methanol,acetone, acetic acid, THF, acetonitrile, chloroform, toluene, hexane,cyclohexane, nitrobenzene, nitromethane, phenol, cyclohexylamine,bibutylamine, aniline, butylamine, triethylamine, hydrazine, andammonium hydroxide. All the compounds and/or solvents (HPLC orspectroscopic grade) were purchased from Fisher or Aldrich, and used asreceived.

UV-vis absorption and fluorescence spectra were measured on aPerkinElmer Lambda 25 spectrophotometer and LS 55 fluorometer,respectively. SEM measurement was performed with a Hitachi S570microscope (operated at 10 kV). The sample was prepared by casting onedrop of the nanofiber suspension in hexane onto a clean glass coverslip, followed by drying in air and then annealing overnight in an ovenat 45° C. The dried sample was coated with gold prior to the SEMimaging. The bright-field optical and fluorescence microscopy imagingwas carried out with a Leica DMI4000B inverted microscope, using aRhodamine filter set, which provides excitation in the range of 530-560nm, and collects emission at >580 nm.

The fluorescence quenching by amines vapor was monitored. Briefly, thefluorescence spectra of the film were measured immediately afterimmersing inside a sealed-jar (50 mL) containing small amount of theamines. To prevent direct contact of the film with the amines, somecotton was placed above the amines (deposited at the bottom of the jar).Before use the jar was sealed for overnight to achieve saturated vaporinside. The presence of cotton also helps maintain a constant vaporpressure. The fluorescence quenching at the diluted vapor pressures ofamines (e.g., aniline and hydrazine) was performed in a sealed cuvette(5 mL volume), into which a small volume of the saturated vapor of aspecific amine was injected (using an air-tight micro-syringe) toachieve the diluted vapor. For example, injection of 5 μL of saturatedaniline vapor (880 ppm) into the 5 mL cuvette will produce a vaporpressure 1000 times diluted, e.g., 880 ppb. The lowest vapor pressure ofaniline that can be achieved through vapor dilution was about 35 ppb,for which two steps of dilution were carried out, i.e., 50 μL of theambient saturated vapor of aniline was injected into a 5 mL jar immersedin a water bath (ca. 70° C., to avoid minimal condensation of thevapor), followed by injecting 20 μL of this diluted vapor into the 5 mLcuvette.

The time-dependent fluorescence quenching profile (shown in FIG. 6) wasmeasured with an Ocean Optics USB4000 fluorometer, which can be switchedto the mode to measure the emission intensity at a selected wavelengthas a function of time. An open sample holder (Ocean Optics, CUV-ALL-UV)was used to hold the nanofibril film deposited on a glass cover slip,and the fluorescence from the nanofibers was collected at 90° withrespect to the excitation beam, which was provided by an Ar⁺ laser(Melles Griot) tuned at 488 nm. Both the excitation and emission weretransported with 0.6 mm premium UV/Vis fibers (Ocean Optics). Thefluorescence quenching was carried out by blowing a few mL of saturatedaniline vapor (880 ppm) onto the nanofibril film during the course whenthe emission was continuously recorded by the fluorometer.

Indeed, as shown in FIG. 6, upon exposure to the saturated vapor ofaniline (880 ppm) the fluorescence of the nanofibril film wasinstantaneously quenched by almost 100%. Such efficient fluorescentsensing was also observed for a broad range of amines (primary,secondary, and tertiary) as listed in Table 1 above. The fluorescencequenching thus observed is due to a photoinduced electron transferprocess as depicted in FIG. 4 where the electron transfer is driven bythe favorable energy difference between the HOMO of aniline and the HOMOof PTCDI (which is now one electron vacant in the excited state). Thehigh efficiency of the fluorescence quenching is consistent with thelarge driving force for the photoinduced electron transfer between theexcited state of molecule 1 and the amine molecules (FIG. 4 and Table1).

To explore the detection limit for some of the representative aminessuch as aniline and hydrazine, the same quenching process shown in FIG.3B was also examined for the diluted amine vapor. FIG. 3A shows thefluorescence quenching efficiency (1−I/I₀) of a nanofibril film measuredat four different vapor pressures of aniline, 1, 1000, 10,000 and 25,000times diluted from the saturated vapor (880 ppm) at room temperature.The quenching data are well fitted to the Langmuir equation with anassumption that the quenching efficiency is proportional to the surfaceadsorption (coverage) of amines. From the fitted plot the detectionlimit of the nanofibril film shown in FIG. 3A can be projected as low as˜200 ppt, if considering the fact that a well-calibrated photodetector(e.g., PMT) can detect intensity change as small as 0.1% or below.Following the same procedure the detection limit for hydrazine wasestimated to be ˜1 ppb.

FIG. 6 shows the emission intensity of the film monitored as a functionof the time after exposed to the saturated vapor of aniline (880 ppm).Fitting the intensity decay into a single exponential kinetics deduces aresponse time for the quenching process (defined as the decay lifetime),only 0.32 s. The fast response thus obtained for the nanofibril sensoris mainly due to the three-dimensional continuous, porous structureformed by the entangled piling of the nanofibers, which allows forexpedient diffusion of the analyte molecules throughout the film matrix,thus leading to instant capture (and accumulation) of the vapor species.The fast sensing response, along with the low detection limits and therobust photostability (zero photobleaching, as shown in FIG. 8)observed, makes the nanofibril film an ideal probing system in a broadrange of applications, particularly for onsite amine monitoring andscreening, where instant vapor detection of trace amines is usuallydemanded.

The nanofibril film also demonstrated high selectivity in response toorganic amines, with minimal fluorescence quenching observed for othercommon organic reagents, such as those listed in FIG. 9. For all theamines tested, more than 85% fluorescence quenching was observed for thenanofibril film upon exposure to the saturated vapor of amines, whereasall the other organic liquids and solids (except for phenol) examined asthe potential background interference exhibited less than 3%fluorescence quenching under the same testing conditions (FIG. 9). Thesignificant quenching (˜54%) observed with phenol is likely due to itsstrong reducing power, i.e., electron-donating capability.

Interestingly, the fluorescence quenching observed with phenol washighly reversible as shown in FIG. 10 (without chemical reaction), wherethe fluorescence of the nanofibril film after exposure to the phenolvapor could be recovered almost 100% simply by re-exposing it toatmosphere for ˜60 min (or at an elevated temperature, e.g., 60° C., foronly 5 min). The recovered film demonstrated the same quenchingefficiency when used in the next cycle of the test with the phenol vapor(FIG. 10). Such a reversible quenching can be used to distinguish phenol(if present) from the organic amines, which otherwise exhibited almostirreversible fluorescence quenching under the same conditions, i.e.,only ˜50% of the fluorescence could be restored even after heating upthe film overnight. The less reversibility observed for the quenchingwith amines is largely due to the much more stable chemical bindingbetween amines and the anhydride moiety of molecule 1. Thus, thereported fluorescence sensor system can be typically provided as asingle-use device, in the similar manner as a pH paper or pregnancy kit,which can be used by ordinary people without worrying about how torecover the materials after each use.

Although the fluorescence of the nanofibers cannot be recovered afterexposed to the amines, the PTCDI materials (molecules) can be recoveredsimply by redissolving the nano fibers into chloroform, followed byappropriate purification (e.g., extraction with water) to remove theamines. The PTCDI molecules thus recovered (showing again the 100%fluorescence quantum yield) can be refabricated into the nano fibers andmaintain the same sensing efficiency for amines. To this end, the PTCDImaterials are recyclable, in contrast to the other irreversible sensorsystems, for which the sensor materials are usually unrecyclable due topermanent chemical damage.

Example 2

This example provides a system with increased sensitivity for aminesover Example 1 (lower detection limit). Ultrathin nanofibers only 30-50nm in diameter were fabricated from a perylene based molecule,N-(1-hexylheptyl)perylene-3,4,9,10-tetracarboxyl-3,4-anhydride-9,10-imide.The ultrathin nanofibers hereby fabricated, in comparison with the muchlarger fibers of Example 1 enables enhancement in fluorescence quenchingefficiency, mainly due to the increased surface area offered by theultrathin nano fibers, which in turn allows for increased vapor exposureto amines. Moreover, films formed from thinner fibers possess increaseporosity, facilitating the expedient cross-film diffusion of gaseousspecies and thus enhancing the collection and accumulation of the tracevapor analytes, combination of which leads to unprecedented sensingsensitivity.

The ultrathin nanofibers were prepared by a quick crystallizationprocess, i.e. directly injecting a good solvent solution of the perylenemonoimide into a poor solvent in a small test tube, followed by aging.FIG. 11A shows a SEM image of the nanofibers measured by a FEI NanoNovamicroscope, demonstrating relatively uniform size and shape withdiameter ranging from 30 to 50 nm. The nanofibers exhibit the same UVabsorption and fluorescence emission spectra as that of the largerfibers (350 nm in diameter), which was fabricated through a vapordiffusion process, i.e. about 0.2 mL CHCl₃ solution of the perylenemonoimide (1.7 mM) was exposed to a methanol vapor in a closed chamberfor one day. The same spectral property (and thus electronic structure)is indicative of the same intermolecular organization for these twosizes of fibril structures despite of the different fabrication methods.This simplifies the comparative study when employing the two sizes offibers for the vapor sensing of amines, for which the fiber size will bethe only major factor determining the sensing sensitivity, rather thanthe molecular stacking mode. The same intermolecular stacking structureof the ultrathin nano fiber also yields the same fluorescence quantumyield as that of the larger fibers (ca. 15%), which facilitates theapplication in fluorescence sensing. FIG. 11B shows a fluorescencemicroscopy image of a nanofibril film deposited on a glass substrate,where strong red fluorescence emission of the nanofibers can easily bevisually observed.

In this example, aniline was chosen as the target vapor analyte, mainlydue to its relatively lower saturated vapor pressure (880 ppm) comparedto other organic amines, which makes it easy to dilute the vapor down toa pressure level that matches the detection limit for the new nanofibrilsensing system as described below. For example, 35 ppb of aniline vaporcan be simply generated by injecting 0.2 mL saturated aniline vapor intoa 5 mL cuvette. This value represents the lowest vapor pressure so farproduced in this lab, and has been used in the test of the fluorescencequenching sensitivity of the ultrathin nanofibers. The fluorescencequenching experiments were performed by injecting the saturated anilinevapor into a sealed optical cell (5 mL) with the nanofibers deposited onone inner surface. The fluorescence spectra of such a nanofibril film(0.35 mg totally deposited) in the presence of different pressures ofaniline vapor are shown in FIG. 12A.

Dramatic fluorescence quenching (13%) was observed for the nanofibrilfilm after 60 s of exposure to only 35 ppb aniline vapor. As calculated,considering both the molecular amount of the nanofibers and anilinevapor, one aniline molecule can quench the fluorescence emissioncorresponding to seven building-block molecules within a nanofiber,i.e., the fluorescence quenching is amplified due to the one-dimensionalenhancement of exciton diffusion along the long axis of nanofiber. Underthe same measurement condition, only ca. 4% quenching (FIG. 13A) wasobserved with the larger nanofibers (350 nm in diameter), i.e. oneaniline molecule can only quench two building-block molecules emission.The decreased quenching efficiency is likely due to the enlargedcross-section size of the fibers, for which the exciton diffusion ismore bulk dispersed, not as confined along the long axis as expected forthe ultrathin nano fibers. One-dimensional confined exciton diffusion isusually conducive to enhancement of fluorescence quenching if theintermolecular energy transfer is dominant along the long axis ofnanofibers. This illustrates an effective way to improve the quenching(sensing) efficiency simply by decreasing the size of the nanofibers,which in turn increase the surface area of the nanofibril film thusdeposited.

It should be noted that the real sensitivity of the nanofibril filmshown in FIG. 12 should be much higher than the measured value if takinginto account the technical fact that the small volume (0.2 μL) ofaniline vapor cannot be released completely into the cuvette due to thesignificant absorption in the syringe. Moreover, the smaller size ofnanofibers are conducive to enhancing the porosity of the film thusdeposited, i.e., producing a smaller pore structure but with a morebulky inter-pore connection. This enhanced porosity, along with theincreased surface area, not only facilitates the adsorption of aminevapor, but also strengthen the accumulation of the amine species thuscollected from the gaseous phase.

Indeed, once the aniline molecules were adsorbed into the nanofibrilfilm, they usually remain condensed within the solid phase, no releaseback to the gaseous atmosphere. This is consistent with the resultspresented in FIG. 12B, where the quenched fluorescence remainedunchanged even 30 min after the film was exposed to 1750 ppb of anilinevapor. In contrast, for the film deposited from the larger fibers(diameter of 350 nm) the fluorescence intensity tended to graduallyincrease after exposure to the same vapor pressure of aniline,indicating significant release of aniline molecules back to the gaseousphase (inset, FIG. 12B). The sustainable accumulation of gaseousanalytes within the film matrix is crucial for enabling trace vaporsensing, for which expedient and effective collection of analytemolecules from the atmosphere environment is often a defining factor forthe sensing system.

Technically, as small as 0.1% (or below) change in fluorescence emissionintensity can be detected by a well-calibrated photodetector (e.g.,PMT). Based on such a photon detection threshold, one way to furtherimprove the vapor sensing sensitivity (or detection limit) is toincrease the signal-to-noise ratio. Generally, the less the nanofibersare employed, the less the quencher molecules are needed for the samepercentage of fluorescence quenching, thereby leading to enhancedsensitivity to the trace vapor analyte. However, to maintain thesufficient adsorption and accumulation for the trace vapor, the filmdeposited from a smaller amount of fibers can maintain a sufficientlyhigh surface area and porosity. To this end, ultrathin nanofibers areideally suited for fabrication as thin films (potentially using muchless materials), while still maintain high surface area and porosity.

FIG. 13A shows the fluorescence quenching in response to the vapor ofaniline measured for the nanofibril films deposited from differentamount of fibril materials. Under the same vapor pressure, largerquenching percentage was observed for the film fabricated with lessamount of nanofibers. For example, under the vapor pressure of 35 ppb,31% of fluorescence quenching was observed for the film deposited from0.15 mg nanofibers, whereas only 13% of fluorescence quenching wasobtained for the film deposited from 0.35 mg nano fibers. Whendecreasing the amount of nanofibers down to 0.1 mg, the quenchingefficiency was further increased to 39% under the same condition. Theincreased quenching efficiency implies direct improvement of thedetection limit.

FIG. 13B shows the fluorescence quenching data fitted with the Langmuirequation. Taking a fluorescence intensity change as 1%, the detectionlevel for the 0.35 mg film is predicted at ca. 1 ppb, whereas for the0.15 mg film the value could be as low as 0.1 ppb. In contrast, for thefilm deposited from 0.35 mg large fibers (350 nm diameter) the detectionlevel is up to 5 ppb. The lower sensitivity thus observed for the largefibers is mainly due to the intrinsic smaller surface area and lowerporosity. If assuming as small as 0.1% (or below) fluorescence quenchingcan be measured by a well-calibrated photodetector (e.g., PMT), thedetection limit for the 0.15 mg film can be as low as ca. 5 ppt.

In conclusion, the fluoresence sensing sensitivity of perylene basednanofibril films for amine vapor was largely enhanced by decreasing thesize of the nanofibers, which were fabricated through a solution-basedself-assembly processing. The enhanced fluorescence sensing is mainlydue to the increased surface area and the enhanced exciton diffusionalong the long axis of nanofiber, along with the increased porosityintrinsic to the film deposited from the ultrathin nanofibers. Thesensing efficiency (or detection limit) can further be enhanced byreducing the amount of the nanofibers employed in the film.

The foregoing detailed description describes the invention withreference to specific exemplary embodiments. However, it will beappreciated that various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theappended claims. The detailed description and accompanying drawings areto be regarded as merely illustrative, rather than as restrictive, andall such modifications or changes, if any, are intended to fall withinthe scope of the present invention as described and set forth herein.

What is claimed is:
 1. An amine sensor assembly comprising a porous filmof entangled nanofibers on a substrate, the nanofibers having a stackednanofiber structure including a 3,4,9,10-tetracarboxyl perylene compoundhaving the formula I:

where A and A′ are independently chosen from N—R1, N—R2, and O such thatboth A and A′ are not O, and R1 through R10 are independently selectedfrom the group consisting of amine binding moieties, solubilityenhancing groups, and hydrogen such that at least one of R1 through R10is an amine binding moiety.
 2. The amine sensor assembly of claim 1,wherein A is N—R1 and A′ is O.
 3. The amine sensor assembly of claim 2,wherein R1 is a C1 to C13 alkyl.
 4. The amine sensor assembly of claim2, wherein R1 is hexylheptyl, pentylhexyl, or butylpentyl.
 5. The aminesensor assembly of claim 1, wherein A is N—R1 and A′ is N—R2.
 6. Theamine sensor assembly of claim 5, wherein at least one of R1 and R2 is aC1 to C13 alkyl.
 7. The amine sensor assembly of claim 5, wherein atleast one of R1 and R2 is selected from the group consisting ofhexylheptyl, pentylhexyl, butylpentyl, COOH, cyclopentyl, cyclohexyl,cycloheptyl, cyclooctyl, and cyclododecyl.
 8. The amine sensor assemblyof claim 5, wherein one or two of R3 through R10 is COOH.
 9. The aminesensor assembly of claim 1, wherein R4 and R5 collectively form maleicanhydride.
 10. The amine sensor assembly of claim 1, wherein thenanofibers have a diameter from about 10 nm to about 1000 nm.
 11. Theamine sensor assembly of claim 10, wherein the nanofibers have adiameter from about 10 nm to about 50 nm
 12. The amine sensor assemblyof claim 1, wherein the amine-binding moiety includes an oxygen moietyor an acid.
 13. The amine sensor assembly of claim 12, whereinamine-binding moiety is an anhydride.
 14. The amine sensor assembly ofclaim 12, wherein the amine-binding moiety is a carboxylic acid.
 15. Theamine sensor assembly of claim 1, wherein R3—R10 contains at least oneamine-binding moiety or solubility enhancing group.
 16. The amine sensorassembly of claim 1, wherein A is N—R1 and A′ is O and R1 is a branchedalkyl or wherein A is N—R1 and A′ is N—R2 and the second amine-bindingmoiety is an anhydride or carboxylic acid and the solubility enhancinggroup is a branched alkyl.
 17. A method of detecting amines in a fluid,comprising: a) exposing the fluorescent sensor compound of claim 1 to afluid sample; and b) displaying a fluorescence change upon exposure ofthe nanofiber sensor compound to the fluid sample.